Effects of the formation of reactive chlorine species on oxidation process using persulfate and nano zero-valent iron

The chloride ion (Cl^-) is a matrix ion that plays crucial roles in radical-based oxidation processes used to treat brackish or saline water. Here, the effects of the formation of reactive chlorine species on the performance of and reaction mechanisms involved in persulfate/nano zero-valent iron process were evaluated by investigating the reaction kinetics and performing reactive species scavenging tests. The phenol oxidation rate increased markedly in the early reaction stage in the presence of 25-200 mM of Cl^-. This was because excess sulfate radicals (SO₄^-) reacted with Cl^- to produce short-lived reactive chlorine species such as Cl and Cl₂^- rather than being scavenged by Fe²⁺ or other SO₄^-. The reactive chlorine species caused OH to form through radical propagation reactions. The total numbers of reactive species involved in phenol oxidation were higher at brackish to weakly saline Cl^- concentrations than at lower and higher Cl^- concentrations. At high Cl^- concentrations (>400 mM), the phenol oxidation rate decreased because most of the SO₄^- reacted with Cl^- to give large amounts of weaker oxidants such as Cl₂^- and HOCl. Acceleration of Fe corrosion by Cl^- negligibly affected the persulfate/nano zero-valent iron oxidation process.